Loop negative feedback is an important method for improving performance of a linear amplifier. For a loop negative feedback amplifier, larger loop feedback depth results in less distortion of the amplifier. However, further increase in feedback depth is limited by stability of the amplifier. That is to say, if the frequency of amplified signal is higher, the loop negative feedback depth obtainable will be smaller. More particularly, for a power amplifier, since it is necessary to utilize power elements of bad frequency characteristic, or use more stages of buffer amplifiers to provide sufficient gain and to accommodate the complex change of output load, signal of higher-frequency will achieve smaller negative feedback depth. For example, the negative feedback depth of the audio power amplifier generally is only 10 times (20 db) at a frequency of 20 KHz. Therefore, the capability of the loop negative feedback to lower amplifier distortion is limited.
There is also a need to solve the negative feedback loop stability problem of the amplifier for purpose of increasing the feedback depth of the amplifier. Instability of the loop feedback is due to: a phase shift of negative 180 degree to the closed-loop feedback at a certain frequency, thus satisfying the condition of generating a positive feedback. In addition, the closed-loop gain at the frequency is larger than 1, and oscillations are generated due to satisfaction of self-excited oscillation.
Accordingly, it is necessary to provide a method for double sampling loop negative feedback and double sampling negative feedback amplifier for overcoming disadvantages of the conventional technology.